1. Field of the Invention
The present invention relates to filters having both hydrophobic (water repellent) and oleophobic (oil repellent) properties. The properties are produced by forming a dimethylsiloxane coating on a substrate such as a hydrophobic or hydrophilic membrane or other filtration medium.
2. Background of the Invention
Hydrophobic filters are used in filtration of gases, in venting filters, and as gas vents. These hydrophobic filters allow gases and vapors to pass through the filter while liquid water is repelled by the filter.
Polytetrafluoroethylene (PTFE) has been the most common material in filters for gas venting. PTFE is chemically and biologically inert, has high stability, and is hydrophobic. PTFE filters therefore allow gases to be selectively vented while being impervious to liquid water.
Hydrophobic membranes are used as filters in healthcare and related industries, for example, as vent filters for intravenous (IV) fluids and other medical devices. In the health industry, the membrane must be sterilized before use. PTFE membranes can be sterilized for these health-related applications with steam or by chemical sterilization without losing integrity.
Treating PTFE membranes with steam can cause pore blockage due to condensation of oil from the machinery used to generate the steam. The resulting loss of air permeability reduces the membrane""s ability to serve as an air vent. Although chemical sterilization minimizes exposure of the membrane to oil, chemical sterilization uses toxic chemicals and can generate byproducts which cause waste disposal problems. Ionizing radiation has also been used for sterilization of materials used in medical and biological devices. PTFE is unstable toward ionizing radiation. Irradiated PTFE membranes have greatly reduced mechanical strength and cannot be used in applications where they are subjected to even moderate pressures.
Perhaps the two biggest drawbacks to PTFE as a filter for venting gases are the high cost and the low air permeability of PTFE membranes. PTFE membranes are formed by extruding and stretching PTFE. Both the PTFE raw material and the processing to form the PTFE membranes are expensive. Further, the extruding and stretching processes used to form PTFE membranes create a membrane which has relatively low air permeability.
The oleophobicity of PTFE can be improved by impregnating or coextruding the PTFE with siloxanes (for example, U.S. Pat. No. 4,764,560), fluorinated urethane (U.S. Pat. No. 5,286,279), or perfluoro-2,2-dimethyl-1,3-dioxole (U.S. Pat. No. 5,116,650). Although the oil resistance of the PTFE is improved, the treated PTFE membranes are expensive, and air permeability remains fairly low.
As a result, efforts have been made to identify alternative substrates which are less expensive and have higher air permeability than PTFE and which can be modified to be hydrophobic and oleophobic.
Coating filtration substrates allows one to retain the desirable bulk properties of the substrate while only altering the surface and interfacial properties of the substrate. Coating substrates to increase their hydrophobic and oleophobic properties has not been very practical, because the coatings can reduce permeability. Furthermore, many of the coating materials are expensive.
Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface of substrates made of nylon, polyvinylidene difluoride (PVDF), and cellulose by treating the substrate with a fluorinated acrylate monomer. The substrate was sandwiched between two sheets of polyethylene, and the monomer was polymerized by exposing to ultraviolet light. The resulting composite filters had hydrophobic and oleophobic surfaces. The air permeability of the filters decreases with time.
Moya (U.S. Pat. No. 5,554,414) formed composite filters from polyethersulfone and PVDF membranes with a method similar to that of Scarmoutzos. The resulting filters did not wet with water or hexane. The disadvantage of the Moya filters is that air permeability of the treated filters was lower than the untreated substrates, and the fluorinated monomer is expensive.
Sugiyama et al. (U.S. Pat. No. 5,462,5856) treated nylon fabric and PTFE membranes with solutions containing two different preformed fluoropolymers. The treated filters were resistant to water and oils. The durability of filters coated with preformed polymers is often less than that for filters where the coating is formed by polymerizing a monomer on the surface of the substrate.
Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a variety of membranes and fabrics with solutions of fluoroacrylate monomers and formed coatings on the substrate by polymerizing the monomer. The coating conferred oil and water repellency onto the substrate. However, the air flow through the treated membrane was reduced, compared to the untreated membrane.
Hydrophobic media suitable for garments have been made by extruding mixtures of polypropylene or PTFE and the fluorochemical oxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media made by extruding tend to have relatively low air permeability.
There is a need for an oleophobic and hydrophobic filter which is inexpensive and has high air permeability. Specifically, there is a need for a coating for filter medium substrates that makes the substrate oleophobic and hydrophobic, and for a more cost effective process of making oleophobic filters.
The present invention provides an oleophobic, hydrophobic, coated filter, including a substrate, the substrate having a pressure of water penetration, the coated filter further including a coating derived from a coating formulation, wherein the coating is permanently crosslinked to the substrate, and wherein the coating formulation includes a vinyl-terminated siloxane polymer, and wherein the coated filter has a pressure of water penetration at least 10 percent greater than the pressure of water penetration of the substrate without the coating. The substrate may include a porous polymeric membrane, a nonwoven material, or a woven material. The substrate may include a polymer such as polysulfone, polyethersulfone, polyarylsulfone, polyvinylidene fluoride, polypropylene, polyethylene, poly(tetrafluoroethylene), poly(tetrafluoroethylene-co-ethylene), nylon, or cellulosic esters. The siloxane polymer may include a vinyldimethyl-terminated siloxane. The coating formulation may further include a crosslinker, such as, for example, methylhydro,cyanopropylmethylsiloxane; methylhydro,phenylmethylsiloxane; methylhydro,methyl-octylsiloxane; methyltriacetoxy silane; or methyl silicone. The coating formulation may further include a crosslinker catalyst. The filter of the invention may also be bonded to a fabric.
In another aspect of the invention, there is provided a method of producing a hydrophobic, oleophobic filter, including the steps of providing a substrate having a first pressure of water penetration; contacting the substrate with a coating formulation including a vinyl-terminated siloxane polymer to produce a coated filter; crosslinking the coating formulation to the filter; and recovering an oleophobic, hydrophobic, permanently coated filter having a second pressure of water penetration, wherein the second pressure of water penetration is at least 10 percent greater than the first pressure of water penetration. In this method, the substrate may include a porous polymeric membrane, a nonwoven material, or a woven material. The substrate may include a polymer such as polysulfone, polyethersulfone, polyarylsulfone, polyvinylidene fluoride, polypropylene, polyethylene, poly(tetrafluoroethylene), poly(tetrafluoroethylene-co-ethylene), nylon, or cellulosic esters. The siloxane polymer may include a vinyldimethyl-terminated siloxane. The coating formulation may further include a crosslinker, such as, for example, methylhydro,cyanopropylmethylsiloxane; methylhydro,phenylmethylsiloxane; methylhydro,methyl-octylsiloxane; methyltriacetoxy silane; or methyl silicone. The coating formulation may further include a crosslinker catalyst. The filter of the invention may also be bonded to a fabric. The crosslinking step may include exposing the coated filter to a temperature sufficient to facilitate a crosslinking activity of the crosslinker. The coating formulation further may include a crosslinker catalyst. The crosslinking step may also include exposing the coated filter to water or water vapor, or exposing the coated filter to ultraviolet radiation.
The present invention relates to hydrophobic and oleophobic filters that have high gas permeabilities and that repel water and other liquids. The invention also relates to methods of preparing such filters.
The filter medium substrate is treated with a coating material comprising crosslinked vinyldimethyl terminated siloxane, which treatment coats the surface of the substrate. Coating the substrate with a material comprising crosslinked vinyldimethyl terminated siloxane gives permanent oleophobicity and hydrophobicity to the filter. The treated filters have high permeabilities for air flow and reject liquid water, as evidenced by high water penetration pressures. The filters are useful, for example, as air filters or vent filters for intravenous (IV) or other medical devices. The critical surface tension for spreading (yc), which is defined as the wettability of a solid surface by noting the lowest surface tension a liquid can have and still exhibit a contact angle (xcex8) greater than zero degrees on that solid, was dramatically reduced after treatment of the substrates according to the process of the invention.
The process can be used to coat substrates made from sulfone polymers such as polysulfone, polyethersuflone, or polyarylsulfone, as well as other polymers, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyolefins including polyethylene and polypropylene, poly(tetrafluoroethylene-co-ethylene) (ECTFE), acrylic copolymers, polyamides, polyesters, and polyurethanes.
For the present invention, substrates may include not only flat sheet polymer membranes made from casting a polymer-solvent-nonsolvent dope mix, but substrates may also refer to any other suitable filtration or exclusion medium. Non-limiting examples of other media within the meaning of substrate include hollow fibers, melt blown or other nonwoven media, woven media, or sedimented structures. In addition, filters of the present invention may be composites, such as, for example, composites having different layers of any of the foregoing media, composites having multiple layers of the same medium, or composites having layers of the same medium, but of different pore sizes, porosities, geometries, orientations, and the like.
In accordance with the invention, the substrate can be coated by any workable method. A few examples of approaches to coat formation are provided herein.
However, the possible useful coating methods are not limited to the methods listed below:
1. Crosslinking the coat formulation to the substrate by moisture curing polydimethyl siloxane with the crosslinker methyltriacetoxy silane.
2. Polymerizing vinyl terminated siloxane (structure I, below) with a crosslinker such as hydrosilicone in the presence of a catalyst. The reaction is shown below.
3. Crosslinking a coating to a substrate by curing a siloxane coating on a substrate by exposure to ultraviolet (UV) radiation.
4. Heat crosslinking with methyl silicone at a temperature above 100xc2x0 C.
In the first method, the substrate is impregnated with polydimethyl siloxane and the crosslinker methyltriacetoxy silane to form a coat on the substrate. The coat is then cured with moisture to further bond the coat to the substrate. The moisture in the air slowly cures the siloxane polymers in a process that may require more than 12 hours. The moisture cure systems can employ the most common crosslinkers such as, for example, acyloxy, enoxy, and oxime crosslinkers.
In the second method of forming the coating, a vinyl terminated siloxane is reacted with a crosslinker such as hydrosilicone in the presence of a catalyst. The structure of the vinyl terminated siloxane is shown below as I: 
The reaction which occurs between the vinyl terminated siloxane and the hydrosilicone is: 
The vinyl terminated siloxane can have one or more vinyl groups. The weight % of vinyl groups can range from 0.1 to 0.4 wt %. The viscosity of the vinyl terminated siloxane can be from 500 to 70,000 centipoise (cps), more preferably from 800 to 10,000 cps, and most preferably from 1,000 to 5,000 cps
The crosslinker is a compound which contains one or more silicon-hydrogen bonds. Hydrosilicone is an exemplary crosslinker. Methylhydro, trimethylsilyl-terminated dimethylsiloxane is another preferred crosslinker. In a preferred embodiment, the crosslinker, hydrosilicone, has viscosity of 25-35 cps and a molecular weight of 500 to 10,000 Daltons. The weight % of the methylhydro units in the polymer is about 15 to 50%. Other crosslinkers, such as methylhydro,cyanopropylmethylsiloxane, methylhydro,methyloctyl siloxane, and dimethylsiloxy-terminated methylhydro,phenylmethyl siloxane also can be used. The silicon-hydrogen bond of the crosslinker reacts with the double bond of the vinyl group of the vinyl terminated siloxane. The weight ratio of vinyl terminated siloxane to crosslinker is between 20 and 0.1, more preferably between 15 and 0.5, and most preferably between 10 and 1.
In many crosslinker/catalyst formulations, the formulation may contain 5-10 ppm platinum, which is preferably in the form of platinum 1,3-diethylenyl-1,1,3,3-tetramethyldisiloxane complexes. Platinum divinyltetramethylsiloxane complex is an exemplary noble metal catalyst for forming the coating of the invention. The noble metal catalyst is normally present in a concentration of 1 to 100 ppm, more preferably 5 to 10 ppm, calculated as the weight of the noble metal catalyst. The noble metal catalyst can be formed in-situ, or it can be formed prior to addition to a reaction solution. Although the noble metal catalyst can be insoluble or soluble in the reaction solution, it is generally preferred that the noble metal catalyst be soluble in the reaction solution. Nonlimiting examples of noble metals are nickel, copper, palladium, silver, platinum, and gold. Other catalysts such as zinc and tin chlorides, zinc acetates, zinc octoates, and peroxides can also be used.
The vinyl terminated siloxane, crosslinker, and noble metal catalyst may be dissolved in a solvent. The solvent may be a hydrocarbon such as, for example, hexane or toluene. The selected solvent should not react with or dissolve the substrate or the crosslinked coating.
The coating formulation containing the vinyl terminated siloxane, crosslinker, noble metal catalyst, and solvent, is contacted with the substrate at a temperature between 15 and 30xc2x0 C. In a preferred embodiment, the contacting takes place at approximately room temperature. The two part crosslinker systems thus may include catalysts and silanol-terminated polymers with a molecular weight of about 26,000 to 200,000 Daltons.
The substrate is soaked in the solution for about 15 seconds to 5 minutes, more preferably 30 seconds to 3 minutes, and most preferably 1 to 2 minutes. The coated substrate is then removed from the coating solution and is air dried for 1 to 180 minutes, then oven cured at a temperature of 100 to 150xc2x0 C., more preferably 105 to 130xc2x0 C., and most preferably 110 to 120xc2x0 C. for 1 to 180 minutes, more preferably 5 to 120 minutes, and most preferably 10 to 60 minutes, to produce the coated filter of the invention.
As a third alternative, crosslinking may be achieved by impregnating the substrate with polydimethyl siloxane and then exposed to ultraviolet (UV) radiation to cure the coating on the substrate. Likewise, in a fourth embodiment, methyl silicone may be impregnated into the substrate to form the coat, and the coat then may be crosslinked by heat curing at a temperature above 100xc2x0 C.
Vinyl-terminated siloxanes or vinyl-terminated fluorosiloxanes may be dissolved in a solvent selected from the group consisting of fluorocarbons, hydrocarbons, and alcohols such as, for example, isopropanol. Preferably, the solvent is not a solvent of the substrate, and can be a hydrocarbon such as, for example, hexane. The solution containing solvent and vinyl-terminated siloxanes and crosslinkers is contacted with the substrate at a temperature of about 15 to 30xc2x0 C. for about 30 seconds to 5 minutes. Preferably, the contacting takes place at approximately room temperature for several seconds to 2 minutes.
Embodiments of the coating process can be used to coat substrates including asymmetric or isotropic membranes, or other media such as, for example, melt blown, woven, and non woven material. Melt blown material may include polypropylene or ECTFE, and are commercially available from U.S. Filter/Filtrate Division, Timonium, MD.
The substrate can be treated with sufficient coating agent so that the coated filter contains at least 0.1 wt % of the coat comprising vinyldimethyl-terninated siloxane, more preferably between 0.5 and 6 wt % coat material, and most preferably 1 to 3 wt % coat material.
The hydrophobic, oleophobic filters of the invention, employing any useful substrate, also can be bonded to a textile fabric or other woven or nonwoven material to form a layered fabric capable of excluding the passage of liquid while allowing passage of vapors and gasses therethrough. Such a layered fabric can be useful in a variety of applications, as will be appreciated by those or ordinary skill in the art. Bonding a hydrophobic, oleophobic filter to a fabric can be accomplished by conventional adhesives, thermal bonding, and the like, and can also be achieved by layering the filtration medium substrate together with the fabric, and curing or otherwise crosslinking the coating formulation thereafter. In this embodiment, the substrate may be coated prior to layering, or the coating may be simultaneously with, or after, the layering of substrate with fabric.